1. Field of the Invention
The invention relates to an X-ray examination apparatus which includes an X-ray detector for receiving an X-ray image and an exposure control system for adjustment of the X-ray examination apparatus on the basis of a relevant part of the X-ray image. The invention also relates to an X-ray examination apparatus provided with an X-ray detector for deriving an optical image from an X-ray image and an exposure control system for adjustment of the X-ray examination apparatus on the basis of a relevant part of the optical image. The invention also pertains to a method of adjusting an x-ray apparatus.
2. Description of Related Art
An X-ray examination apparatus of this kind is known from U.S. Pat. No. 5,461,658.
The X-ray examination apparatus includes an X-ray source for irradiating an object to be examined, for example a patient to be radiologically examined, by means of an X-ray beam. Due to local differences in the X-ray absorptivity within the patient, an X-ray image is formed on an X-ray-sensitive surface of the X-ray detector. The X-ray detector derives an image signal from the X-ray image. The image signal is, for example an electronic video signal whose signal levels represent brightness values of the X-ray image. The known X-ray examination apparatus includes an X-ray image intensifier for deriving an optical image from the X-ray image. The known X-ray examination apparatus also includes a television camera for deriving the electronic video signal from the optical image. The X-ray image has a large dynamic range, being the interval comprising the brightness values of the X-ray image. However, relevant image information in the X-ray image is comprised in a small range within the much larger range of brightness values of the entire X-ray image. If no steps were taken, the range of the values of the signal level of the image signal would not be suitable for suitably visible reproduction of the image information in the X-ray image. Particularly the dynamic range of brightness values of the X-ray image, and hence of the optical image, would be much too large for the further processing of the image signal.
The known X-ray examination apparatus includes an auxiliary light detection system which acts as an exposure control system. The auxiliary light detection system includes a CCD sensor for locally measuring the brightness in the optical image. The exposure control system derives a control signal from the measured brightness values, said control signal being used to adjust the X-ray apparatus in such a manner that an X-ray image of high diagnostic quality is formed and displayed, i.e. that small details are included in the X-ray image and suitably visibly reproduced. The auxiliary light detection system adjusts the X-ray examination apparatus in such a manner that signal levels of the image signals have values which are suitable for reproducing an image of high diagnostic quality. The control signal controls the intensity and/or the energy of the X-ray beam. The control signal can also be used to control the amplification of the image signal. Both steps influence the signal level of the image signal directly or indirectly.
The auxiliary light detection system of the known X-ray examination apparatus utilizes local brightness values in the optical image in order to adjust, for example the X-ray source, but it does not always take into account the fact that overexposed areas of high brightness occur in the optical image. Such overexposed areas are caused, for example by X-rays which are not or only hardly attenuated by the object to be examined, for example a patient. In that case X-rays are involved which have not passed through the patient or have traversed tissue having a low X-ray absorptivity, for example lung tissue. Such overexposed areas contain hardly any or even no image information, but could have an adverse effect on the adjustment of the known X-ray examination apparatus. The auxiliary light detection system of the known X-ray apparatus also does not take into account the fact that dark areas of low brightness which do not contain relevant image information either may also occur in the X-ray image. For example, X-ray absorbing elements such as collimator elements or filter elements are reproduced as such dark areas in the X-ray image. It has been found that the overexposed areas and dark areas, both of which do not contain any or hardly any relevant image information, can have an adverse effect on the adjustment of the known X-ray examination apparatus.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.